The effect of Helisoma duryi on the cercarial production of Schistosoma mansoni-infected Biomphalaria alexandrina: evaluation of chemical interferences and direct competition.

The effect of Helisoma duryi or uninfected Biomphalaria alexandrina conditioning of water, as well as the effect of direct competition with these, on the total cercarial production, growth and survival of Schistosoma mansoni-infected B. alexandrina was investigated. The total cercarial production varied considerably between experimental groups. No differences were found between groups maintained in water-conditioned by H. duryi or by uninfected B. alexandrina with regard to the total cercarial production, growth and survival. The cercarial production was correlated with snail size, but showed no differences between the two treatments. Direct competition with H. duryi resulted in reduced cercarial production and growth of infected B. alexandrina as compared with snails in competition with uninfected conspecifics.